The invention relates to a piezoelectric flexural transducer comprising an elongated support body which on at least one longitudinal side is fitted with a piezoelectric unit, which has a piezoelectric body with one or more piezoelectric material layers and electrodes associated with same, the electrodes being connected in an electrically conducting manner with the support body. Furthermore, the invention relates to use of such a piezoelectric flexural transducer.
A prior art piezoelectric flexural transducer of this type in accordance with the patent publication WO 99/17383 possesses an elongated support body, which at opposite longitudinal sides is fitted with a respective piezoelectric unit. The piezoelectric unit comprises a piezoelectric body manufactured using multi-layer technology and possessing a plurality of piezoelectric material layers and electrodes associated with same. By the application of a drive voltage to the electrodes electric fields may be produced in the piezoelectric body, this causing a longitudinal contraction of the piezoelectric body and furthermore a deflection, comparable with a pivoting motion, of the piezoelectric flexural transducer in relation to the position at which the piezoelectric flexural transducer is held. In order to be able to apply the drive voltage to the electrodes of a respective piezoelectric unit are in electrical contact with the support body, which is able to be connected with a source of voltage.
During operation of such known flexural transducers moisture may engender problems. If the piezoelectric flexural transducer is for example employed as the setting member of a valve for controlling fluid flow, and is at all times surrounded by pressure medium containing moisture, it is possible for partial short circuits to occur, which impair the deflection characteristics of the piezoelectric flexural transducer and the functional reliability of the means fitted with the piezoelectric flexural transducer.
One object of the present invention is to provide a piezoelectric flexural transducer of the type initially mentioned which while allowing simple manufacture is not sensitive to external effects and more particularly is insensitive to moisture. A further aim resides in the use of such a piezoelectric flexural transducer.
The first mentioned object is to be attained by a piezoelectric flexural transducer of the type initially mentioned in the case of which the piezoelectric unit comprises an electrically insulating coating surrounding and encasing the piezoelectric body, such coating being provided with one or more electrically vias for making electrical contact with the electrodes.
It is in this manner that the invention provides a piezoelectric flexural transducer, in the case of which the piezoelectric body having the electrodes is completely encapsulated by the electrically insulating coating surrounding it and is screened off from the outside. The piezoelectric body and its component parts are therefore protected against external influences and functional impairment such as that due to moisture, is not possible. For the application of the drive voltage the piezoelectric unit is consequently provided, in accordance with its particular design, with one or more electrically conductive vias extending through the coating.
It would in principle be possible to provide the entire piezoelectric flexural transducer with a uniform coating. However, the separate coating of one or more already existing piezoelectric units independently of the support body has substantial advantages both from the point of view of manufacturing technology and also as regards costs. The vias can produce the electrically conductive connection between the electrodes of the piezoelectric material and the conductors of the support body or lead to electrodes on other outer sides of the piezoelectric material.
Further advantageous developments of the invention are defined in the dependent claims.
It is convenient for the vias to be located in the section of the coating, facing the support body, so that on the application of a piezoelectric unit to the support body electrical contact between a respective piezoelectric unit and the support body is made simultaneously. Furthermore, the adhesive layer normally present between the piezoelectric unit and the support body and serving for securing the piezoelectric unit may reliably seal off the surrounding part of the vias from the outside.
At each respective via the support body preferably possesses an electrically conductive contact face connected with the via. It is also an advantage for each electrically conductive via to be provided with a contact layer on the outer face (which faces the support body) of the section, which rests on the support body, of the coating, such layer serving to make contact with the support body. It may be made with a sufficiently large periphery so that no complex adjustment work is necessary for mounting the piezoelectric unit with a reliable and secure contact making effect.
The structure in accordance with the invention is also advantageous even when the piezoelectric body has only one piezoelectric material layer, that is to say is made in the form of a monolithic piezoelectric body. Owing to the plurality of electrodes present such structure is however particularly to be recommended in the case of a multi-layer piezoelectric body with a plurality of piezoelectric material layers.
As a material for the electrically insulating coating it is preferred to employ a parylene (generic term for thermoplastic polymer films, which are formed from p-xylene) or a polyimide (generic term for high temperature stable plastics, which generally contain aromatic groups).
The vias may be formed in any desired manner. In the case of one particularly simple design through holes are machined in the coating during the manufacture thereof and such holes are then filled with a conductive material, as for instance a conductive adhesive.
In principle the support body may be provided with its own coating. Since in the form of the piezoelectric body however the relevant components are also encapsulated, a coating-free design of the support body is readily possible as well.
The piezoelectric flexural transducer may as a unimorphous flexural transducer be provided with only one piezoelectric body or as a trimorphous flexural transducer with two encapsulated piezoelectric bodies arranged on opposite longitudinal sides.
To achieve the above mentioned further aim the use of the piezoelectric flexural transducer as a setting member having a drive or sealing function is provided for, for example in conjunction with a valve serving for fluid control. The outer face of the coating may here directly serve as the actuating face and/or sealing face and in this connection may consist of a material with the desired coefficient of friction and/or sealing properties.
In the following the invention will be explained in detail with reference to the accompanying drawings.